In the fields of acoustic pianos, the so-called silent piano has been commercialized that is configured to be provided with a stopper for stopping the striking of strings by hammers so as to output an electronic sound instead of a string-striking sound. The electronic sound is generally output via a headphone or the like. The electronic sound is also output by using the vibration of the soundboard, in order to make the output electronic sound more closely resemble an acoustic piano sound. For example, Patent Literature 1 discloses a technology by which a vibrator that vibrates a soundboard of an acoustic piano is used to produce, from the soundboard, sound that has been generated by an electronic sound source.